Solar cells are photovoltaic (PV) devices that convert light into electrical energy. Solar cells have been developed as clean, renewable energy sources to meet growing demand. Solar cells have been implemented in a wide number of commercial markets including residential rooftops, commercial rooftops, utility-scale PV projects, building integrated PV (BIPV), building applied PV (BAPV), PV in electronic devices, PV in clothing, etc. Currently, crystalline silicon solar cells (both single crystal and polycrystalline) are the dominant technologies in the market. Crystalline silicon (cSi) solar cells must use a thick substrate (>100 um) of silicon to absorb the sunlight since it has an indirect band gap. Also, the absorption coefficient is low for crystalline silicon because of the indirect band gap. The use of a thick substrate also means that the crystalline silicon solar cells must use high quality material to provide long minority carrier lifetimes to allow the carriers to diffuse to the p-n junction. Therefore, crystalline silicon solar cell technologies lead to increased costs. Thin film photovoltaic (TFPV) solar devices based on amorphous silicon (a-Si), CIGS, CdTe, CZTS, etc. provide an opportunity to increase the material utilization since only thin films (<10 um) are generally required. The thin film solar cells may be formed from amorphous, nanocrystalline, micro-crystalline, or mono-crystalline materials. TFPV devices may have a single device configuration (i.e. they are comprised of a single light conversion device) or they may have a tandem configuration wherein multiple TFPV devices are used to increase the absorption efficiency within different wavelength regions of the incident light spectrum.
TFPV devices provide an opportunity to reduce energy payback time, and reduce water usage for solar panel manufacturing. The absorption coefficient for CIGS is about 105/cm. CIGS films have bandgaps in the range of 1.0 eV (CIS) to 1.65 eV (CGS) and are also efficient absorbers across the entire visible spectrum. Among the thin film solar technologies, CIGS has demonstrated the best lab cell efficiency (over 20%) and the best large area module efficiency (>15%).
A general class of PV absorber films of special interest is formed as multinary compounds from Groups IB-IIIA-VIA of the periodic table. Group IB includes Cu, Ag, and Au. Group IIIA includes B, Al, Ga, In, and TI. Group VIA includes 0, S, Se, Te, and Po. Additionally, the IB-IIIA-VIA materials can be doped with dopants from Groups VIII, IIB, IVA, VA, and VIIA of the periodic table. Group VII includes Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt. Group IIB includes Zn, Cd, and Hg. Group IVA includes C, Si, Ge, Sn, and Pb. Group VA includes N, P, As, Sb, and Bi. Group VIIA includes F, CI, Br, I, and At. Other potential absorber materials of interest include cuprous oxide, iron sulfide, etc.
TFPV devices can be fabricated at the cell level or the panel level, thus further decreasing the manufacturing costs. As used herein, the cell level is understood to mean an individual unit that can be combined with other units to form a module. The cells may be rigid or flexible. As used herein, the panel level is understood to mean a large TFPV structure that is not composed of smaller units. Generally, the panels are similar in size to the aforementioned modules. For economy of language, the phrase “TFPV device” will be understood to refer to either a solar cell or a panel without distinction. Furthermore, TFPV devices may be fabricated on inexpensive substrates such as glass, plastics, and thin sheets of metal. Examples of suitable substrates comprise float glass, low-iron glass, borosilicate glass, flexible glass, specialty glass for high temperature processing, stainless steel, carbon steel, aluminum, copper, polyimide, plastics, etc. Furthermore, the substrates may be processed in many configurations such as single substrate processing, multiple substrate batch processing, in-line continuous processing, roll-to-roll processing, etc.
The increasing demand for environmentally friendly, sustainable and renewable energy sources is driving the development of large area, thin film photovoltaic devices. With a long-term goal of providing a significant percentage of global energy demand, there is a concomitant need for Earth-abundant, high conversion efficiency materials for use in photovoltaic devices. A number of Earth abundant direct-bandgap semiconductor materials now seem to show evidence of the potential for both high efficiency and low cost in Very Large Scale (VLS) production (e.g. greater than 100 gigawatt (GW)), yet their development and characterization remains difficult because of the complexity of the materials systems involved.
The complexity of TFPV devices exploiting Earth abundant materials represents a daunting challenge in terms of the time-to-commercialization. That same complexity also suggests an enticing opportunity for breakthrough discoveries. A quaternary system such as CIGS requires management of multiple kinetic pathways, thermodynamic phase equilibrium considerations, defect chemistries, and interfacial control. The vast phase-space to be managed includes process parameters, source material choices, compositions, and overall integration schemes. The complexity of the intrinsically-doped, self-compensating, multinary, polycrystalline, queue-time-sensitive, thin-film absorber (CIGS), and its interfaces to up-, and down-stream processing, combined with the lack of knowledge on a device level to address efficiency losses effectively, makes it a highly empirical material system. The performance of any thin-film, (opto-)electronically-active device is extremely sensitive to its interfaces. Interface engineering for electronically-active devices is highly empirical. Traditional R&D methods are ill-equipped to address such complexity, and the traditionally slow pace of R&D could limit any new material from reaching industrial relevance when having to compete with the incrementally improving performance of already established TFPV fabrication lines, and continuously decreasing panel prices for more traditional cSi PV technologies.
However, due to the complexity of the material, cell structure and manufacturing process, both the fundamental scientific understanding and large scale manufacturability are yet to be improved for CIGS TFPV devices. As the photovoltaic industry pushes to achieve grid parity, much faster and broader investigation is needed to explore the material, device, and process windows for higher efficiency and a lower cost of manufacturing process. Efficient methods for forming different types of CIGS TFPV devices that can be evaluated are necessary.
Typically, CIGS is formed by the selenization of Cu—In—Ga precursors that have been previously deposited. The selenization is typically performed in a batch furnace using H2Se gas as the source of Se. A common problem encountered in this process is the Ga segregation towards the Mo back contact due to the different reaction kinetics of In and Ga with H2Se. This produces a non-uniform band gap in the depth of the absorber. The Ga poor surface layer has a low band gap that limits the open circuit voltage (Voc) of the solar cell. In addition, the lattice mismatch caused by the large inhomogeneity in depth can introduce structural defects and recombination centers in the space charge region, which adversely impacts device performance. For large area CIGS thin film modules, high Voc is important for performance improvement because series resistance losses need to be minimized. A reasonable criterion for high efficiency modules is a Voc greater than 600 mV. Another benefit of high band gap solar cells is improved performance in hot climates due to favorable temperature coefficients. Therefore, there is a need to develop systems and methods for the selenization of Cu—In—Ga films that allow the production of high performance CIGS films with control over the Ga distribution.